This invention relates generally to non-complimentary or single-ended noise reduction systems and more particularly concerns the release response characteristics of dynamically controlled filtering circuits used in such systems. A complete dynamic noise reduction system is disclosed in my U.S. Pat. No. 4,696,044. This system has a linear release response characteristic per period of time. Due to the fact that the voltage control filter control law is linear in volts per decade of frequency, the release of this system is relatively linear in time versus decade of filter cut-off frequency. This means that the release time for the filter cut-off to change from 20 KHz to 10 KHz is approximately the same as the release time from 1 KHz to 500 Hz. With a linear release time there is a compromise between audible breathing with a slower release and a lack of ambiance with a fast release.
With typical program material, high-level high-frequency information such as sibilance on vocals is present for a very short period of time, on the order of a few milliseconds, whereas background ambiance and reverb can trail on for up to a few hundred milliseconds. Typically, the high-frequency decay of reverb is very fast. However, the long reverb tail which continues for hundreds of milliseconds contains much mid-frequency information from 500 Hz up to 2 KHz. If a release rate for the dynamic filter is selected to reduce any audible breathing in the high frequency spectrum, i.e., 50 ms for the bandwidth to change from 32 KHz to 4 KHz or 3 decades of frequency, then the mid-band release from 4 KHz to 500 Hz or 3 decades of frequency will also be 50 ms. This will reduce the transparency of the noise reduction system by greatly reducing the perceived openness of the audio signal. Use of a single-pole 6 dB per octave filter helps reduce this effect by providing a gradual cut-off attenuation slope. 12 or 18 dB per octave filtering would obviously provide much more noise reduction capabilities. In the original design, only a small portion of the RC release curve is used and this provides a somewhat linear release or voltage versus time response. It is possible to improve system transparency by exploiting the full exponential release characteristics of a simple RC circuit. With a simple RC circuit designed to provide a 50 ms release from 32 KHz to 4 KHz, the release time from 4 KHz to 500 Hz increases to approximately 200 ms. While this is an improvement it still requires a design compromise between breathing, transparency and filter cut-off slope. To eliminate this compromise, it is desirable to provide a dynamic filter with a non-linear release such that the release time in each specific portion of the audio spectrum is optimized for maximum transparency.
It is, therefore, an object of the present invention to provide a dynamic noise reduction system with a frequency-bandwidth-dependent release having a very fast filter release in the high-frequency spectrum, a moderate release in the upper mid-frequency spectrum, and a very slow release in the lower mid-frequency portion of the audio spectrum. Yet another object of the present invention is to provide a filter control circuit that allows 12 to 18 dB per octave filtering without sacrificing audible transparency or sonic quality.